Semiconductor devices are typically formed as multilayer structures in which the semiconductor materials are present in a patterned array that defines channels for the transport of charge carriers. For example, an inorganic semiconductor layer may be applied to a dielectric substrate surface. A mask may then be applied to protect regions of the semiconductor layer, intended to constitute charge carrier channels, from a subsequently applied etchant. The etchant then removes the semiconductor in the unmasked regions, leaving behind a finely patterned array of semiconductor channels on the substrate. In the absence of such patterning, the semiconductor devices may be inoperable or be subject to excessive crosstalk.
Inorganic semiconductors typically are rigid and brittle at ambient temperatures. Hence, semiconductor devices formed with inorganic semiconductors generally are rigid as well. As the myriad end use applications for semiconductor devices have evolved, availability of semiconductor devices that can be flexed and bent without damage is desirable. Flexible semiconductor device structures also offer potential capability for bulk processing in the fabrication of large area device arrays, such as a continuous web for example, at low unit costs.
Much work has been done to develop organic semiconductors for applications where flexible semiconductor devices are needed. However, organic semiconductors generally cannot survive the harsh conditions required in order to carry out an etching step to generate a patterned array of channels for charge carrier transport. Printing processes have accordingly been sought in order to directly provide a patterned semiconductor channel array without a need to remove regions of a continuous layer of material. Unfortunately, the fine feature definition that is needed to generate microarrays of semiconductor channels has not been attained by printing of organic semiconductors.
Pentacene, for example, has become an organic semiconductor of great interest due to its high conductivity when formed into a crystalline film. Single dendritic crystals of pentacene having dimensions as large as 2 millimeters long, 1 millimeter wide, and 0.5 millimeter thick have been produced. For example, individual thin film field effect transistors comprising single pentacene crystals have been made with high channel mobilities within a range of between about 1 centimeter squared per volt-second (cm2/Vs) and about 5 cm2/Vs, at room temperature. However, these individual thin film field effect transistors often have low on/off current ratios, and are not integrated into a circuit.
Pentacene is known to have high conductivity when formed on substrates comprising poly(vinylphenol). However, direct patterning of pentacene leads to poorly defined and irregular channel boundaries that cannot be used in fabricating semiconductor devices such as transistors or arrays comprising transistors.
One effort to generate a suitably patterned array of semiconductor channels comprising pentacene involved the direct printing of a pentacene precursor, which was then converted into pentacene. See, for example, Dimitrakopoulos et al. U.S. Pat. No. 5,981,970, entitled, “Thin-film field-effect transistor with organic semiconductor requiring low operating voltages.” However, the performance of semiconductor devices made by this process was unsatisfactory. For example, the process required a high temperature annealing step that degraded the semiconductor devices. Further, two different material patterning methods were needed. The stability of the resulting semiconductor devices was also unreliable.
Another process for forming devices utilizing patterned organic semiconductor films, disclosed in Katz U.S. Pat. No. 6,403,397 issued on Jun. 11, 2002 and entitled “Process For Fabricating Organic Semiconductor Device Involving Selective Patterning,” involved treating a surface to selectively provide regions of greater affinity and lesser affinity for an organic semiconductor or an organic semiconductor solution. When the organic semiconductor, or solution comprising the semiconductor, was deposited on the treated surface, either the organic semiconductor or the organic semiconductor solution dewetted from the lesser affinity regions or the resultant film adhered only weakly to the lesser affinity regions such that selective removal was readily performed. Even where such removal was not performed, the portions of the organic semiconductor film overlying the greater affinity regions exhibited higher conductivity and better film continuity relative to the other portions of the film.
There remains a need for semiconductor devices comprising semiconductors having finely patterned regions of high and low conductivity. There further is a need for methods of making semiconductor devices employing semiconductors that are not easily patterned.